An IQ modulator or quadrature modulator in the form of a vector modulator is normally used to modulate a carrier frequency in a transmission arrangement using a frequency modulation method. In this case, the actual transmission signal containing the modulation data is usually produced, during digital signal processing, at an intermediate frequency which is low or disappears. Corresponding transmission structures are referred to as low IF or zero IF transmitters. In this case, IF is an abbreviation for intermediate frequency. The IQ mixer in the form of a double-balanced mixer is used to step up this modulated signal (in the form of a single-sideband signal) to the desired carrier frequency. In a quadrature modulator of this type, both the modulation signal containing the actual modulation data and the carrier signal must be broken down into quadrature components. Quadrature components are also referred to as IQ components. In this case, the in-phase and quadrature components are orthogonal to one another and form a complex signal.
A control loop in the form of a phase locked loop (PLL) is usually provided in the transmission architecture for the purpose of controlling the transmission frequency. A PLL of this type comprises, for example, a phase detector, a loop filter, a charge pump, a voltage-controlled oscillator and a frequency divider, which are appropriately interconnected. The unmodulated carrier signal, rather than the modulated output signal from the transmission arrangement, is normally fed to the phase locked loop for frequency control in order to prevent low-frequency components of the modulation signal from being eliminated by the phase locked loop.
However, the principle described has the disadvantage that, when modulating a frequency component before the frequency of the oscillator signal is converted to the transmission frequency, only an unmodulated frequency component, and therefore not the final frequency or output frequency of the transmitter, can be fed to the phase locked loop.